bsd-user: fix compile failure

bsd-user: fix compile failure

bsd-user doesn't actually support reserving a memory area for the
guest address space, but we need to at least define the reserved_va
global so that cpu-all.h's RESERVED_VA macro will work correctly.

This fixes a compilation error introduced in commit 39879bb
which added a use of RESERVED_VA to h2g_valid().

Reported-by: Brad Smith <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Stefan Weil <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>